Remove cast in place chimney liner or replace chimney.

[Abc]

My wife and I am building a masonry heater from a kit, but my current chimney flue is too small. It is a 6 inch cast in place liner and we need an 8 in flue. The current concrete block chimney will accommodate an 8 inch stainless liner if we can figure out how to remove the current liner. Is this possible? Local chimney sweeps don't seem to know how.

If we can't remove the liner we're looking at removing and replacing the entire thing. Are there any thoughts on pros and cons of class a stainless vs concrete block. It is a 25 foot tall chimney running through the middle of our 2 story house. I am a carpenter not a mason, but I do have some friends with experience laying block who would be able to help.

Ease of install vs cost are the two big contrasts I see between stainless steel and masonry chimneys.

Thanks!

 

[EbS-P]

Does the current chimney meet code with 2” of clearance to all combustibles?

I’m guessing not since they did a pour in place insulation on a 6” liner. Clearance to combustibles may be reduced to zero if the liner is insulated. If you don’t have room for and 8” insulated liner then your options are. Tear down or figure out if you can get the needed 2” clearance an go un insulated 8”.

 

[Monkeysocks]

I have a similar setup, two story, interior and if it’s the cast in place I’m thinking of (volcanic rock pour for both insulation and structural integrity.)
Unfortunately there is no way to get it out short of chimney demolition. That is by design because when it is poured the slurry seeps into any cracks or voids and added support to the chimney. I’ve spent a lot of time think about how to remove since I could use the space it takes up to run one chimney to the basement for wood furnace and one to main floor for stove.

I would recommend knocking out and replacing with stainless class A.
Pros
- time and effort, way quicker than masonry build since you will already have walls opened up for the chimney demo.
- better insulation and hotter flue gases. Design of masonry heaters is low exhaust temps
- cost when everything is factored in. Back to the low exhaust temp, you would want an insulated liner in the masonry chimney, add that to block and mortar cost and you’re probably around the same as the Class A.
- Removes thermal bridge outside envelope. Admittedly this isn’t one a lot of people consider, but might be a factor with a masonry heater. Concrete block is a bad insulator so where it exits the roof isn’t exactly an open window, but it’s pretty close.